1. Technical Field
The present invention relates to a conductive member evaluator which evaluates a conductive member charging an electrostatic latent image carrier provided in an electrophotographic image forming apparatus or the like and a conductive member evaluation method.
2. Description of the Related Art
An electrophotographic image forming apparatus such as a copier, a laser beam printer or a facsimile includes a charging member which performs a process of uniformly charging a surface of a photoreceptor as an electrostatic latent image carrier (i.e., image carrier) on which an electrostatic latent image is formed. As this charging member, a conductive member formed in, for example, a roller shape (hereinafter, referred to as a charging roller) is used. A contact charging method which performs the above charging process by bringing the outer circumferential face of the charging roller into contact with the surface of the photoreceptor is conventionally known (for example, Japanese Patent Application Publication No. H01-267667).
However, the above-described contact charging method has the following problems. (1) The substances constituting the charging roller leak from the charging roller, and adhere to the surface of the photoreceptor. (2) Vibration sound is caused by applying AC voltage to the charging roller. (3) The charging performance is deteriorated if toners on the photoreceptor adhere to the charging roller. (4) The permanent deformation of the contact portion between the charging roller and the photoreceptor is caused by stopping the rotation of the photoreceptor for a long period of time.
In order to solve the above problems, a close charging method, which performs a charging process by closely disposing the charging roller to have a predetermined gap (space) to the surface of the photoreceptor, is proposed (for example, Japanese Patent Application Publication No. H05-107871). In such a close charging method, the charging roller faces the photoreceptor such that the closest distance of the gap becomes 50-300 μm, and the voltage in which the DC voltage and the AC voltage are superimposed is applied to the charging roller, for example, so as to perform the charging process to the photoreceptor. Since the charging roller does not have contact with the photoreceptor in this close charging method, the above problems caused in the contact charging method can be solved.
The charging roller for use in the above contact charging method is required to uniformly have contact with the surface of the photoreceptor in order to uniformly charge the photoreceptor. For this reason, the circumference of the cored bar of the charging roller is coated by an elastic vulcanized rubber. On the other hand, the charging roller for use in the close charging method is required to maintain a uniform gap between the charging roller and the photoreceptor in order to uniformly charge the photoreceptor. For this reason, the circumference of the cored bar of the charging roller is coated by a non-elastic thermoplastic resin without having aging deterioration and deformation. As described above, the charging roller for use in the contact charging method is made of a material different from that of the charging roller for use in the close charging method.
It is known that the discharge in accordance with Paschen's Law in the minute discharge between the charging roller and the photoreceptor significantly contributes to the charging mechanism to the surface of the photoreceptor by the above-described charging roller, regardless of the contact charging method or the close charging method. Therefore, it is important to evaluate the electric property (discharge feature) of the charging roller in order to evaluate the basic performance of the charging roller which uniformly charges the surface of the photoreceptor at predetermined potential.
As a method of evaluating an electric property of a charging roller (i.e., a conductive member evaluation method), the following charging roller evaluation method is used. In this charging roller evaluation method, as schematically illustrated in FIG. 14, a plurality of bar-like electrodes 302 is connected to an outer circumferential face 101a of a charging roller 101 in a resting state in the axial direction, an electrode 303 is connected to a cored bar 106 of the charging roller 101, DC voltage is applied between the electrodes 302, 303 by a DC power source, a resistance value of the charging roller 101 is calculated by measuring the current value flowing in the charging roller 101, and the charging roller 101 is evaluated by using this resistance value. According to this evaluation method, a charging roller having a lower resistance value is evaluated as a charging roller which has a high conductive performance, enables even discharge and can obtain a preferable image.
As another conductive member evaluation method, Japanese Patent Application Publication No. 2009-300899 and Japanese Patent Application Publication No. 2010-72056 describe the following method. In this conductive member evaluation method, the above-described charging process is performed while rotating in opposite directions a charging roller and a photoreceptor having a length substantially the same as the length of the charging roller, which are disposed in parallel with a contact state or a close state, similar to an actual image forming apparatus, the current waveform flowing in the charging roller and the photoreceptor is measured, and the charging roller is evaluated based on the area of the measured current waveform and the phase difference between the applied voltage waveform and the measured current waveform.
Various configurations, manufacturing methods or the like have been developed for the above-described charging rollers. However, an image formed by these charging rollers may include local uneven concentration depending on the configuration, the manufacturing method or the manufacturing condition of the charging roller. FIG. 15 illustrates one example of an image including local uneven concentration. The right and left direction in FIG. 15 conforms to the axial direction of the charging roller and the up and down direction conforms to the circumferential direction of the charging roller. In this description, “local” means a part of the axial direction in the outer circumferential face of the charging roller or a part of the direction corresponding to the axial direction and a part of the circumferential direction or a part of the direction corresponding to the circumferential direction.
The following is considered as a reason for such local uneven concentration. For example, when forming an electric resistance adjusting layer on a cored bar as a shaft center in manufacturing a charging roller, an adhesion property between the cored bar and the electric resistance adjusting layer is locally deteriorated due to an extrusion molding method, an injection molding method or a condition of the molding, so that the electric property such as a resistance value in this portion changes relative to another portion; thus, the conductive performance becomes uneven. Namely, a local difference of an electric property occurs in the outer circumferential face of the charging roller, so that local uneven concentration occurs in an image by this difference.
However, in the conventional evaluation method using the device illustrated in FIG. 14, since a plurality of bar-like electrodes 302 is connected to the outer circumferential face 101a of the charging roller 101 in the axial direction, the resistance values are measured in a plurality of portions of the outer circumferential face 101a in the axial direction. Consequently, the resistance value can not be measured in a part of the axial direction. Since the charging roller 101 is measured in a resting state, a plurality of electrodes 302 is moved in the circumferential direction every measurement, so that the charging roller 101 can not be continuously measured in the circumferential direction. Namely, an electric property can not be measured in a local part of the outer circumferential face of the charging roller. For this reason, a local difference of the electric property in the outer circumferential face of the charging roller can not be detected, and the local uneven concentration of the image can not be evaluated.
Moreover, in the another conventional evaluation method using an area of a current waveform and a phase difference between a voltage waveform and a current waveform, since the entire charging roller in the axial direction discharges to the photoreceptor, the discharge can not be performed to a local portion of the outer circumferential face. For this reason, similar to the above evaluation method, the local uneven concentration of the image can not be evaluated.